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How do you know if your rear end is going bad?

I have an 88 IROC with the L98. I have the 2.77 rear end gear (RPO GH3) with posi-traction (RPO G80).

When going around corners under moderate acceleration, there is a subtle oscillation (about 0.5 second frequency) that feels like the power is being transmitted back-n-forth between the two rear wheels. This is a relatively new behavior.

Is my posi unit going bad? This is the first car that I have had with a posi unit so I don't have any experience spotting trouble or working with posi units.

Well, I had a trusted mechanic do that about 2,000 miles ago. He said everything looked good. Even so, I had him change the reatr end gear fluids for me. Since that time, I've autocrossed the car in 5 events and I am wondering if I haven't pushed it beyond it's capability...